TR AC HE AT A. 



333 



Graber (No. 412) for various Hymenoptera (Apis), Diptera (Chironomiis), 

 Lepidoptera and Coleoptera (Melolontha, Una). 



Considerable variations in the development of the enveloping mem- 

 branes are known. 



When the fold which gives rise to the membranes is first formed, there 

 is, as is obvious in fig. 181 A, a perfectly free passage by which the yolk 

 can pass in between the amnion and serous membrane. Such a passage of 

 the yolk between, the two membranes takes place posteriorly in Hydro- 

 philus and Donacia : in Lepidoptera the yolk passes in everywhere, so that in 

 this form the ventral plate becomes first of all imbedded in the yolk, 

 and finally, on the completion of the dorsal integument, the embryo is 

 enclosed in a complete envelope of yolk contained between the amnion 

 and the serous membrane. During the formation of the dorsal integument 

 the external yolk sack communicates by a dorsally situated umbilical canal 

 with the yolk cavity within the body. On the rupture of the amnion 

 the embryo is nourished at the expense of the yolk contained in the ex- 

 ternal yolk sack. 



In the Hemiptera and the Libellulidse the ventral plate also becomes 

 imbedded in the yolk, but in a somewhat different fashion to the Lepido- 

 ptera, which more resembles on an exaggerated scale what takes place in 

 Hydrophilus. 



In the Libellulidje (Calo- 

 pteryx) there is first of all 

 formed (Brandt, No. 403) a 

 small ventral and posterior 

 thickening of the blastoderm 

 (fig. 182 A). The hinder 

 part of this becomes in- 

 folded into the yolk as a 

 projection (fig. 182 B), which 

 consists of two lamina?, an 

 anterior and a posterior, 

 continuous at the apex of 

 the invagination. The whole 

 structure, which is com- 

 pletely imbedded within the 

 yolk, rapidly grows in length, 

 and turns towards the front 

 end of the egg (fig. 182 C). 

 Its anterior lamina remains 

 thick and gives rise to the 

 ventral plate (ps), the pos- 

 terior (am) on the other 

 hand becomes very thin, and 

 forms a covering correspond- 

 insr with the amnion of the 



o 



more ordinary types. The 

 remainder of the blastoderm 

 covering the yolk (se) forms 

 the homologue of the serous 

 membrane of other types. 



FIG. 182. THREE STAGES IN THE DEVELOPMENT 

 OF THE EMBRYO OF CALOPTERYX. (After Brandt. ) 

 The embryo is represented in the egg-shell. 



A. Embryo with ventral plate. 



B. Commencing involution of ventral plate. 



C. Involution of ventral plate completed. 



ps. ventral plate; g. edge of ventral plate; am. 

 amnion; se. serous envelope. 



